Viavi 87339, Руководство по эксплуатации

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Principals of Operation
6 -17
6.1.4.H Atmospheric Delays
Ionosphere:
The ionosphere is an atmospheric layer situated between 90 to 1000 km above the
Earth’s surface. The gas molecules in the ionosphere are heavily ionized. The ionization
is caused mainly by solar radiation, hence the thickness of this layer varies during the
course of the day. Signals from the satellites travel through a vacuum at the speed of
light. However, in the ionosphere the velocity of these signals slows down due to the
ionized gas, an effect called dispersion, which is frequency dependent.
Fig. 6-24 Atmospheric Delays
Ionospheric dispersion is one of the most significant error sources. These effects are
smallest when the satellite is directly overhead and become greater for satellites nearer
the horizon as the signal passes through the ionosphere at a shallow angle, hence a
thicker band has to be traversed. Two methods can be used to correct for Ionospheric
delays.
•
Once the receiver's approximate location is known, a mathematical model can be
used to estimate and compensate for these errors.
• Because ionospheric delay affects the speed of microwave signals differently
based on frequency, a second carrier frequency, L2 can be used to measure
atmosphere dispersion and apply a more precise correction to help compensate
for this error. This is the method employed in military GPS receivers using L1 and
L2.
This can also be realized in more expensive civilian GPS receivers without
decrypting the P(Y) signal carried on L2 by tracking the carrier wave instead of
the modulated code. To do this on lower cost receivers, a new civilian code signal
on L2 called L2C was added to the satellites. This new signal allows a direct
comparison of the L1 and L2 signals u sing the coded signal instead of the carrier
wave.
1000 km
90 km
28,000 km
1000 km
90 km
1000 km
90 km
28,000 km